The invention relates generally to methods and apparatus for sensing parameters of sheet material and, more particularly, to sensing multiple parameters of sheet material manufactured in a continuous moving web.
In materials manufacturing, it is often desired to monitor one or more parameters of the product being manufactured. It is particularly important in paper manufacturing to maintain a constant awareness of thickness and moisture content of the web on the paper making machine. It is also desirable to provide a continuous measurement of basis weight of the paper; that is, the weight per unit area.
Many methods are known in the prior art for providing such information. For example, it is known that moisture content in a moving paper web can be measured by microwave techniques as shown, for example, in U.S. Pat. No. 4,484,133. As disclosed therein, a signal related to moisture content and derived from microwave energy can be corrected for thickness of the paper.
It is also known in the prior art to provide various methods for monitoring the thickness of the web. These techniques include the use of microwaves to measure thickness of a moving sheet as disclosed, for example, in U.S. Pat. No. 3,737,770. However, none of the methods known in the prior art, including the aforementioned United States patents, provide satisfactory means for simultaneously generating a spatial profile of moisture and thickness across the width of the web. It would therefore be desireable to provide such simultaneous spatial profile.
It is also known in the prior art to provide a measurement of basis weight. However, such prior art basis weight measurement techniques do not provide satisfactory means for generating an indication of absolute basis weight across the width of the web simultaneously with the production of a similar range of measurements for thickness and moisture. Furthermore, prior art techniques for generating measurements of parameters across the face of the web often involve complex moving-part scanning arrangements. Accordingly, it is desirable to provide apparatus and methods for simultaneously generating spatial profiles of moisture, thickness, and basis weight which eliminate the need for movable components of sensing apparatus.